Rinse the clams thoroughly under cold running water. Drain them well. 2. Place the clams into a large pan. Add 500ml of water to the pan. 3. Cover the pan with a lid. Bring the water to a rolling boil over high heat. 4. Reduce the heat to a simmer. Cook the clams for 2 minutes. They should open during this time. 5. Carefully pour the pan's contents (clams and liquid) into a colander. Place the colander over a large bowl to catch the clam stock. 6. Allow the clams to cool enough to handle. Remove the clams from their shells. You may keep a handful of empty shells for serving, if desired. 7. Strain the clam stock from the bowl into a separate jug. Leave any grit at the bottom of the bowl. You should have approximately 800ml of clam stock. 8. Use the same large pan. Melt the butter over medium heat. 9. Add the bacon to the pan. Cook for 3 to 4 minutes until it begins to brown. 10. Stir in the finely chopped onion, thyme sprigs, and bay leaf. 11. Cook gently for 10 minutes, stirring occasionally. The onion should become soft and golden. 12. Sprinkle the plain flour over the ingredients in the pan. 13. Stir the flour into the mixture to create a sandy paste. Cook for an additional 2 minutes, stirring constantly. 14. Gradually pour in the strained clam stock, stirring constantly to prevent lumps. 15. Gradually add the milk, stirring constantly. 16. Gradually add the double cream, stirring constantly. 17. Add the diced potatoes to the pan. 18. Bring the mixture to a gentle simmer over medium heat. 19. Cook gently for 10 minutes, or until the potatoes are tender when pierced with a fork. 20. Use a fork to crush a few of the potato chunks against the side of the pan. This will help thicken the chowder. Ensure most potato chunks remain whole. 21. Stir in the shelled clam meat. Add the reserved empty clam shells, if you chose to keep them. 22. Simmer for 1 minute to reheat the clam meat. 23. Season generously with black pepper. Add a little salt if needed, tasting first. 24. Stir in the chopped parsley just before serving. 25. Ladle the clam chowder into bowls or hollowed-out crusty rolls and serve hot.